a) Field of the Invention:
The present invention relates to a process for forming ohmic electrodes to a diamond film so as to obtain ohmic electrode-diamond film contacts each having the small contact resistance.
b) Description of the Related Art:
Diamond has high hardness and high thermal conductivity, and also has an excellent stability against heat, radiation and chemicals. It has been known that diamond films can be considerably easily formed by vapor phase synthesis using a chemical vapor deposition (CVD) apparatus. The thus formed diamond films have been used as coatings for speaker diaphragms, heat sinks for ICs and so on. Meanwhile, diamond is an electrical insulating material but can exhibit the semiconducting property by being doped with boron (B) or the like. The B-doped semiconducting diamond has a band gap larger than that of silicon (Si), silicon carbide (SiC) or the like, and hence is expected to be applicable in a high temperature region as compared with Si, SiC or the like. In order to apply the diamond having such properties to heat sinks for ICs and high temperature ICs, there has been required a technique for forming ohmic electrodes to an undoped diamond film or semiconducting diamond film, that is, ohmic diamond-electrode contacts.
The following two techniques have been known to form ohmic contacts to diamond. According to the first conventional technique, electrodes of carbide forming metals such as tantalum (Ta) are formed on the surface of a bulk semiconducting diamond by vacuum deposition, and are then heated by electron beams in vacuum to form ohmic contacts to diamond (A.T. Collins and others, Diamond Research, p. 19, 1970). Further, in place of the heating by electron beams, the semiconducting diamond formed with electrodes on its surface, that is, the whole of the sample is heated in a vacuum chamber to form ohmic contacts (Metallization of Semiconducting Diamond, K. L. Moazed and others, Material Research Society, Boston, 1989). According to the first conventional technique, while the mechanism for forming ohmic contacts being not necessarily clear, it is considered that carbide is formed at the interface between the semiconducting diamond surface and the carbide forming metal electrode by heating, to thereby reduce the contact resistance.
According to the conventional second technique, Ar ions are irradiated with an acceleration voltage of several keV on a semiconducting diamond at the surface areas to be formed with electrodes, and electrodes of Au are formed on the Ar ion irradiated areas, to form ohmic contacts (C.B.Child and others, Fourth Annual SDIO/IST-ONR, Diamond Technology Initiative Symposium, Lecture No. ThP2, Crystal City, Va, 1989). According to this second technique, the surface area to be formed with each electrode in the semiconducting diamond is transformed into graphite, to thereby reduce the contact resistance.
However, in the first conventional technique, since carbide is formed at the interface between the diamond and each carbide forming electrode to form the carbide, the kinds of electrode materials are limited to the carbide forming metals. Further, in the second conventional technique, since the surface area of the diamond to be formed with each electrode is merely transformed into graphite, reduction in contact resistance is restricted.